Antigens can be proteins, polysaccharides, nucleic acids, or lipids that elicit an immune response. Antigens are recognized by B cell receptors or T cell receptors when complexed with MHC. Antigenicity refers to the ability of an antigen to bind antibodies, while immunogenicity is the ability to induce an immune response. Antibodies are Y-shaped glycoprotein molecules composed of light and heavy chains that bind to antigens with high specificity and affinity. There are five classes of antibodies (IgG, IgM, IgA, IgD, IgE) that have different structures and functions in the immune response.

1.  ANTIGEN AND ANTIGENICITY.
 ANTIBODY, STRUCTURE OF ANTIBODY,
TYPES OF ANTIBODY(IMMUNOGLOBULIN
CLASSES), THEORIES OF ANTIBODY
FORMATION.
JAYANTA SUBHASH TIPLE
Guest lecturer, COFSU.
(AQUATIC ANIMAL HEALTH
MANAGEMENT)

2. ANTIGENS
 Antigen can be defined as any substance which, when introduced into the body,
evokes the immune system & reacts with the products of immune system in a
specific & observable manner
 Substance that can be recognized by immunoglobulin receptor of B cells, or by the
T-cell receptor when complexed with MHC, are called antigens.
 Antigens could be proteins (synthetic polypeptides, lipoproteins, and
glycoproteins),polysaccharides (including lipopolysaccharides), nucleic acids or
lipids.
 It also includes parts of coats, capsules, cell walls, flagella, fimbriae, and toxins of
bacteria, viruses, and other microorganisms as well as non-microbial such as
pollen, egg white, etc.

4. TYPES OF ANTIGEN

5. TYPES OF ANTIGEN
 Microbial antigen : These are structural antigens of microbes.
e.g. – Somatic O, Flagellar H, Capsular K, Fimbrilar antigen
 Tissue antigen :Blood group antigen, Transplantation antigen
 Exogenous antigen : Antigens present outside the cell.
e.g. – allergens (pollen, dust), parts of microbes, drugs, pollutants.

6. ANTIGENECITY
 Antigenicity is the ability of an antigen to combine with the final products of the
humoral and/or cell mediated immune response.
 It is not activating the immune response rather it combines with the final products of
the immune response.
 Immunogenicity refers to the ability of an antigen to induce an adaptive immune
response.

7. PROPERTIES OF
ANTIGENS OR
IMMUNOGENS

9.  1. Foreignness
An antigen must be a foreign substances to the animal to elicit an immune response.
 2. Molecular Size
Size should be atleast >10000D
The most active immunogens tend to have a molecular mass of 14,000 to 6,00,000 D.
Insulin (5700D ) are either non-antigenic or weakly antigenic.
 3. Chemical Nature and Composition
The more complex the substance is chemically the more immunogenic it will be.
Antigens are mainly proteins and some are polysaccharides.

10.  4. Physical Form
In general particulate antigens are more immunogenic than soluble ones.
Denatured antigens are more immunogenic than the native form.
 5. Degradability
Antigens that are easily phagocytosed are generally more immunogenic.

11. HAPTENS
 A hapten is a substance that is non-immunogenic but which can react with the
products of a specific immune response.
 It is a small molecule which can elicit an immune response only when attached to a
large carrier such as a protein, the carrier may be one which also does not elicit an
immune response by itself.
 Haptens have the property of antigenicity but not immunogenicity.
 Examples: Drugs like penicillin
Steroid hormone
peptide hormone

13. EPITOPE
 It is the unique region on an Ag that will bind a complementary Ab i.e. that portion
of an antigen that combines with the products of a specific immune response.
 The part of an antibody that recognizes the epitope is called a paratope.
 A single antigen often contains numerous epitopes.
 Forces characteristic of Ag-Ab binding include Van Der Waal’s force, hydrogen
bonding, hydrophobic interaction.
 The other portion of the Ag other than the Epitope are termed immunogenic carrier.

14.  Based on structure and interaction with the paratope. Epitopes divided into:
1)Conformational epitopes
2) Linear epitopes
 A conformational epitope is composed of discontinuous sections of the
antigen's amino acid sequence.
 These epitopes interact with the paratope based on the 3-D surface features and
shape or tertiary structure of the antigen.
 Linear epitopes interact with the paratope based on their primary structure.
 A linear epitope is formed by a continuous sequence of amino acids from the
antigen.

17.  Antibodies are Globulin Protein (Immunoglobulin) that are synthesized
in the Serum and Tissue fluids.
 It reacts specifically with the antigen that stimulated their production.
Antibodies
 The most important function of the Abs are to confer protection against
microbial pathogens.
 Abs confer protection in the following ways:
• They prevent the attachment of microbes to mucosal surface of the host.
• They reduce the virulence of microbes by neutralizing the toxins and
viruses.
• They activate complement, leading to complement- mediated activities
against microbes

18. BASIC STRUCTURES:
 Antibodies have more than one antigen combining site Some bivalent Ab
molecules can combine to form multimeric Abs that have upto 10
combining sites
 All Ig have a basic structure composed of 4 polypeptide chains connected
to each other by disulphide bonds.
 Each light chain consist of 220 aa and has a mass of approx. 25kDa.
 Each heavy chain consists of about 440 aa and has a mass of 50-70kDa.
 Both light and heavy chains contain 2 different regions constant and
variable region
 The four chains are arranged in the form of a flexible “Y” with the hinge
region and is termed as crystallizable fragment (Fc) and contains the site
at which Ab binds.
 Top of the “Y” consist of two Ag binding fragments (Fab) that bind with
antigenic determinant sites.

19. LIGHT CHAIN:
 The light chain may be either of two distinct forms called
“Kappa” and “Lambda” and can be distinguished by aa sequence
of carboxyl portion of the chain.
HEAVY CHAIN:
 In the heavy chain NH2 terminal has a pattern of variability
similar to that of kappa and lambda of the light chain.

20. Structure of Antibody

21. There are five classes of antibodies are present
1) IgG 2) IgM 3) IgA 4) IgD 5) IgE
1)IgG
They makes up approximately 80% of the serum antibodies
They has a half-life of 7-23 days • IgG is a monomer and has 2-epitope binding sites
Functions :-
i. Immunity to new born
ii. Neutralisation of Toxins
2)IgM
They makes up approximately 13% of the serum antibodies • They has a half-life of
about 5 days
Most of the IgM are pentamer and has 10 - epitope binding sites. some are monomer It
is the first immunoglobulin class produced in a primary response to antigen functions:-
i. Activation of classical pathway
ii. Defence against multivalent antigens

22. 3)IgA
They makes up approximately 6% of the serum antibodies • They has a half-life of
approximately 5 days
IgA is a dimer and has 4-epitope binding sites
They found mainly in body secretions such as saliva, mucous, tears, colostrum and milk
Functions :-
i. It as a Secretory antibody
ii. ii. Effective against virus that causing Influnza
4)IgD
They makes up approximately 0.2% of the serum antibodies • IgD is a monomer and
has 2-epitope binding sites
This class antibodies are found on the surface of B-lymphocytes
Function:-
i. B cell activation.
ii. Act a receptor for antigen binding.
5)IgE
It was discovered in 1966 by K. Ishizaka.
It is very low concentration in blood(17-450ng/ml)
It contain small percentage of Lympocytes
Functions :-
i. Responsible for Immediate hypersensitivity
ii. Binds to Fc receptor on basophils and mast cells.

23. REFERENCES
 Research gate
 https://www.researchgate.net/publication/313343795_Immunology
_Lecture_Notes_Antigens_and_Antigenicity
 https://www.researchgate.net/post/All_immunogens_are_antigens
_but_not_all_antigens_are_imunogens
 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6963682/
(Structure and Function of Immunoglobulins- NCBI)
 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2604596/(Theories
of Antibody Formation – NCBI)
 https://www.researchgate.net/publication/281547828_Immunoglob
ulins classes )
 ICAR- Ecources (Fish Immunology)